The invention relates to plasma ion generators and neutron sources based on plasma ion generators, and more particularly to the production of ultra-short pulses from these ion generators and neutron sources.
In many applications, such as time of flight measurements, ultra-short neutron pulses (pulse width<1 μs) with fast rise times or fall times are desired. These neutrons can be high energy, epithermal, thermal, or cold neutrons, and they are normally produced by a fission reactor or an accelerator-based neutron generator. When ultra-short pulses are needed, the neutron output flux can be chopped by means of a rotating mechanical chopper.
There are some disadvantages when these mechanical chopper schemes are used to form ultra-short neutron pulses. First, a large percentage of neutrons will be discarded and activation of material may occur. Second, when pulsed accelerator systems are employed, the mechanical chopper and the ion beam acceleration have to be properly synchronized. Ultra-short pulses cannot be formed by manipulating the plasma discharge because the rise time due to plasma buildup is typically on the order of a few μs.
Other neutron sources are based on ion generators. Conventional neutron tubes employ a Penning ion source and a single gap extractor. The target is a deuterium or tritium chemical embedded in a molybdenum or tungsten substrate.
University of California, Lawrence Berkeley National Laboratory has produced a number of compact neutron sources with a relatively high flux, particularly sources which generate neutrons using the D—D reaction instead of the D–T reaction. These sources have a variety of different geometries, including tubular, cylindrical, and spherical, and are based on plasma ion sources, particularly multicusp plasma ion sources, with single or preferably multiple beamlet extraction. These neutron sources are illustrated by copending U.S. patent applications Ser. Nos. 10/100,956; 10/100,962; and 10/100,955.